Once a mobile device is turned on, the mobile device searches, finds and selects a cell of the chosen public land mobile network (PLMN), turns to its control channel and camps on it. As long as the mobile device is power on, this cell selection and reselection is a continuous process. Especially, when the mobile device moves among cells. Inevitably, the mobile device has to disconnect its communication with the fading cell coverage and reconnect with a new suitable cell so that the mobile device can have a better signal. Based on connected mode radio measurements and cell reselection criteria, the cell reselection process of a mobile device includes selecting a more suitable cell and associated radio access technology (RAT). When a mobile device is in idle mode, URA_PCH, CELL_PCH or CELL_FACH state, it regularly performs cell reselection procedures to search for and camp on a better cell (i.e. cell has higher signal level) according to the cell reselection criteria.
FIG. 1 shows the states and procedures for cell reselection process in URA_PCH, CELL_PCH and CELL_FACH states of connected mode. Assume that the mobile device is in the Universal Mobile Telecommunication System (UMTS) Terrestrial Radio Access (UTRA) Radio Resource Control (RRC) connected mode 5, when a cell reselection is triggered 10, the mobile device starts to evaluate the cell reselection criteria based on radio measurements. And if a better cell using the same RAT is found, then that cell is selected. The mobile device goes back to the UTRA RRC connected mode 5. If the change of cell implies a change of RAT, the existing Radio Resource Control (RRC) connection is released, and the mobile device enters idle mode of the other RAT 20. If no suitable cell is found in the cell reselection procedure, the mobile device will go into the Cell Selection when leaving connected mode 25 and eventually enters an idle mode. Meanwhile when the radio link failure is triggered, the mobile device shall trigger the initial cell reselection procedure 15 in order to request re-establishment of the RRC connection. If the mobile device is unable to find a suitable cell for a period, the mobile device eventually enters an idle mode. If the mobile device finds a suitable cell, the mobile device goes back to the UTRA RRC connected mode 5.
On the other hand, when the mobile device successfully selects a suitable Universal Terrestrial Radio Access (UTRA) cell, the mobile device performs a cell reselection procedure and submits the CELL UPDATE message for transmission on the Uplink Common Control Channel (CCCH) of the selected cell.
According to the prior art, two timers, T314 and T315, are relevant to track either the radio link failure process or the Radio Link Control (RLC) unrecoverable error process in CELL_DCH state of connected mode. The value of timers T314 and T315 are broadcasted in the system information by network, and all the broadcasted timers value shall be stored at mobile device. All the established Radio Access Bearers (RABs), except signalling Radio Access Bearers (RABs) that are used to transfer signalling messages, shall be associated with one of the timers T314 or T315 by network, depending on how long the disconnection can be tolerated by the applications/services using this RAB. When only signalling Radio Access Bearers (RABs) are established, i.e. only RRC connection exists, these signalling RABs are associated with one of the timers T314 and T315. A radio link failure happens when the mobile device receives a plurality of consecutive “out of sync” signals from the physical layer, the mobile device starts the timer T313. If the mobile device receives successive “in sync” signals from its physical layer before the T313 expires, then the mobile device will stop and reset the T313 timer. If the T313 expires, then the mobile device enters a “Radio link failure” state, which the mobile device will clear the dedicated physical channel configuration and perform a cell update procedure. On the other hand, a Radio Link Control (RLC) unrecoverable error happens when the retransmission of a message has exceeded the maximum retries that invokes the RLC unrecoverable error procedure. Upon the triggering of the radio link failure or the unrecoverable error occurrence, either or both timers T314 or T315 will start to count. When both timers T314 and T315 expire, the mobile device will enter an idle mode.
However, according to the prior art, when the mobile device initiates a Cell Update procedure in CELL_DCH state due to a radio link failure or a RLC unrecoverable error, the mobile device of the prior art shall (1) start timer T314, if a Radio Access Bearer (RAB) associated with T314 is established and if the stored value of the timer T314 is greater than zero; or (2) start timer T315, if a RAB associated with T315 is established and if the stored value of the timer T315 is greater than zero; or (3) start timer T314, if there are no RAB associated with T314 nor any RAB associated with T315 is established, i.e. only signalling radio bearers are established, and if the stored value of the timer T314 is greater than zero. When the invoked timer(s) T314 and/or T315 are expired, the mobile device enters an idle mode. Before the invoked timer(s) T314 and/or T315 are expired, the mobile device shall perform Cell Update procedure to try to re-establish the failed radio link or the unrecoverable RLC connection. In addition, when the mobile device initials a Cell Update procedure in CELL_DCH state, if the stored value of both timers are equal to zero, the Cell Update procedure will release all its radio resources, inform the upper layers of the released connections and bearers, clear a plurality of variables and enter an idle mode. Moreover, if the stored value of the timer T314 is equal to zero, the Cell Update procedure will release all radio access bearers associated with T314. Similarly, if the stored value of the timer T315 is equal to zero, the Cell Update procedure will release all radio access bearers associated with T315.
Nevertheless, several undefined or not clear defined scenarios exist during the mobile device's Cell Update operation in the CELL_DCH state if a radio link failure or a RLC unrecoverable error happens. First, when the timer T314 is greater than zero (T314>0) with no associated RAB is established and the timer T315 is equal to zero (T315=0) with at least one associated RAB is established. Second, when T315 is greater than zero (T315>0) with no associated RAB is established and T314 is equal to zero (T314=0) but with at least one associated RAB is established. And last, when T315>0 with no associated RAB is established and T314=0 with no associated RAB is established. In the above three situations, if a radio link failure happens or a RLC unrecoverable error occurs, neither timer T314 nor T315 will be started when the mobile device initiates a Cell Update procedure in the CELL_DCH state. Therefore, if no suitable cell can be found, the mobile device will keep searching a suitable cell until the battery power is exhausted, since there exists no timer started to make the mobile device enters idle mode while expiring.